The disclosure relates to the field of transparent, essentially colorless and non-diffusing β-quartz glass-ceramics. More particularly it relates to transparent, essentially colorless and non-diffusing glass-ceramics of the lithium aluminosilicate type, containing a solid solution of β-quartz as the main crystalline phase, free of As2O3 and of Sb2O3, articles comprising said glass-ceramics, and lithium aluminosilicate glasses as precursors of such glass-ceramics, as well as methods for elaborating said glass-ceramics and said articles.
The disclosed glass-ceramics exhibit very interesting optical properties, e.g., in terms of transmission, non-diffusion and yellow index. Moreover, they have low thermal expansion and are easily obtained insofar as their precursor glasses have low viscosity at high temperatures, viscosities at liquidus compatible with their forming method, and ceramming cycles of short duration.
The Applicant has been producing and marketing since the early 1990's, β-quartz glass-ceramics from precursor lithium aluminosilicate glasses having low thermal expansion coefficients (<10×10−7 K−1 between 25 and 700° C., e.g., between −3 and +3×10−7 K−1 between 25 and 700° C.). Such glass-ceramics may be sued, for example, as cooking top plates, cooking utensils, microwave oven plates, chimney windows, fireplace inserts, fireproof doors and windows, stove and oven windows, notably with pyrolysis or catalysis, and as shields such as transparent armor. The glass-ceramics may be colored, more or less transparent (e.g., black cooking top plates) or transparent, non-colored (e.g., shields, fireproof doors and windows, and cooking top plates for induction heating, optionally with colored lower layers which are desirably perfectly visible).
For obtaining such glass-ceramics (more specifically for removing gas inclusions from the precursor molten glass mass) fining agents such as As2O3 and/or Sb2O3 have been used for a long time. The use of these fining agents is notably illustrated in U.S. Pat. Nos. 4,438,210 and 5,070,045. For instance, U.S. Pat. No. 4,438,210 teaches that Ti—Fe interactions would be responsible for the appearance of a coloration upon ceramming and that the absence of MgO would give the possibility of avoiding this coloration; this, in the presence of As2O3.
With view to the toxicity of As2O3 and of increasingly strict regulations in effect, this toxic fining compound is desirably no longer used. For environmental considerations, it is also desired to no longer use Sb2O3 and not to use halogens, such as F and Br, which would have been able to replace at least partly said fining agents As2O3 and Sb2O3.
SnO2 was proposed as a replacement fining agent. It is increasingly used to this day. It is notably timely used when the precursor glass of the glass-ceramic (glass plate precursors of glass-ceramic plates, in fact) is obtained by floating. Indeed, applied with glasses containing As2O3 and/or Sb2O3 in their composition, such a floating method generates glass plates with a metal deposit at their surface (a metal deposit resulting from the reduction of As2O3 and/or Sb2O3). U.S. Pat. Nos. 6,846,760 and 8,053,381 thus describe the obtaining of glass-ceramics from glasses, themselves obtained by floating and the composition of which contains SnO2 as a fining agent.
The use of SnO2 as a fining agent however has two major drawbacks. This compound is less efficient than As2O3 (and, in absolute terms, it should therefore be used in a relatively large amount, which is not without posing any problems, more particularly of devitrification) and, as a more powerful reducing agent than As2O3 and Sb2O3, it is responsible for the appearance of an undesirable yellowish coloration during ceramming. This second drawback is of course a real nuisance when it is sought to obtain transparent, essentially colorless glass-ceramics.
This yellowish coloration results from Sn—Fe, Sn—Ti and Ti—Fe interactions, i.e., by way of charge transfer. One skilled in the art is actually aware that glass-ceramic precursor glass compositions inevitably contain iron and generally TiO2 as a nucleation agent.
In order to limit or even avoid this yellowish coloration phenomenon, which is quite undesirable, within a context for preparing transparent, essentially colorless glass-ceramics, two different approaches have been suggested to this day.
The first includes adding into the composition of the precursor glass at least one complementary coloring agent or compensating dye, which may also be described as a discoloration agent. The use of Nd2O3 for this purpose was described in U.S. Pat. No. 8,053,381 already mentioned above. Glass-ceramic plates containing this dye, in a substantial amount, are to this day marketed by Schott (DE) under the trademark Pyran® Platinum. The use of expensive dyes of this type, however, is detrimental to the transmission of glass-ceramics. It inevitably causes a loss of a few percents of the integrated transmission.
The second approach, in order to limit or even avoid any Sn—Ti and Ti—Fe interaction, includes limiting or even avoiding the presence of TiO2 within the composition of the precursor glasses.
Both of these alternatives—presence of TiO2 at a limited level and absence of TiO2—were respectively described in commonly-assigned US Patent Application Publication Nos. 2010/0167903 and US 2010/0099546. They also have drawbacks. Compositions without TiO2 or with a limited TiO2 content require ceramming treatments of long duration. Moreover, compositions without TiO2 possess very high liquidus temperatures and are difficult to produce at an industrial scale.
JP patent application 2001-316132 discloses transparent β-quartz glass-ceramics of the lithium aluminosilicate type. The disclosed materials have a thermal expansion coefficient approximating the silica, as well as a high UV transmittance. Such glass-ceramics are more particularly suitable for case material of a device of optical communication such as an optical coupler. Their composition contains 50 ppm or less of Fe2O3. Such a requirement is very severe and implies the use of raw materials having a high degree of purity. Obtaining such a low level of iron is very difficult and expensive under industrial conditions.